For printing a recording medium in one or more colours, for example a single sheet or a strip-like recording medium made of different materials, it is known that on a potential image carrier such as a photoconductor, depending on the image, potential images or charge images can be produced, which correspond to the images to be printed, consisting of areas to be inked and areas not to be inked. The areas to be inked, which are also referred to as image areas, of the potential images are made visible by toner with a developer station (inked). Then the toner image is transferred to the recording medium or substrate.
For inking the image areas, either dry toner or a liquid developer which contains the toner dispersed in a liquid can be used here.
A method for electrophoretic liquid development (electrographic development) is already known; see EP 0 756 213 B1 or EP 0 727 720 B1. A non-polar carrier liquid is used as the liquid developer in the prior-art methods, with toner particles present therein in dispersed form. Typical non-polar solutions which are used as the carrier liquid comprise in general silicone oils and hydrocarbons.
An apparatus which contains a developer station consisting of one or more developer rollers for wetting the image carrier element with liquid developer according to the potential images on the image carrier element, is used in the printing process. By means of one or more transfer rollers, the developed potential image is then transferred to the recording medium.
In the known printing processes using a liquid toner, the toner is embedded in a carrier liquid which transports the toner, but without itself contributing to inking. In addition to the above-mentioned silicone oils and hydrocarbons, mineral oils are also used as the carrier liquid. In the usual printing process, the carrier liquid is transferred together with the toner in a printing unit from an inking station to a photoconductor with the latent printed image via a transfer roller onto a substrate or recording medium.
Transfer of the toner to the substrate is ensured by a layer of the carrier liquid between a transfer roller and the substrate. Driven by electrical forces, the toner migrates through the carrier liquid layer from the transfer roller to the substrate. Thus electrophoretic migration of the toner to the substrate takes place.
For effective transfer of the toner, complete wetting of the surface of the substrate by the carrier liquid is necessary, so that the toner can reach the surface of the substrate and does not remain behind on the transfer roller.
With the known method of this kind, however, there is the problem that part of the carrier liquid remains on the substrate after transfer of the toner to the substrate, and the substrate becomes wet. Therefore the substrate with the toner transferred onto it must be dried by evaporation of the carrier liquid. The usual carrier liquids which are indicated above have the drawback, however, that they have high vaporisation points, so that high temperatures are necessary for drying; for example, temperatures of more than approximately 120° C. are usual.
It is known from U.S. Pat. No. 3,856,519 that the carrier liquid can be replaced during the printing process by another one which evaporates at a lower temperature. For this purpose the transfer surface is wetted before actual transfer with a solution consisting of a highly volatile compound. An electrostatic field is applied in order to cause transfer of the toner image to the substrate surface, preventing the high-boiling carrier liquid such as for example the silicone oil from passing to the substrate. However, a method of this kind is very expensive, and wetting of the substrate with the high-boiling carrier liquid is not always completely avoided satisfactorily.
To ensure prevention of explosion, it is necessary to pass large quantities of scavenging air through the heated section and to heat it. This requires a long drying section and increases the energy consumption.
A further drawback of the method described using the carrier liquid for example based on silicone oils lies in that the carrier liquid is lost due to the necessary drying operation and so always has to be topped up again for a further printing process. This is disadvantageous particularly in view of the costs of the printing method.
The problem to be solved by the invention lies in providing a method for an electrophoretic printing process which is cheap and easy to carry out. This invention is intended to provide a toner the use of which in this printing process does not necessitate expensive evaporation of the costly carrier liquid after transfer. Similarly, the carrier liquid is to be reusable for a further printing process.